Summary: The p53 tumor suppressor gene is mutated or inactivated in a majority of human cancers. p53 is a multifunctional protein that regulates gene transcription and has been implicated in the control of a number of cellular processes including differentiation, apoptosis, growth arrest, and DNA repair. The ability of p53 to bind DNA and transactivate gene transcription was thought to be critical to its tumor suppressor function. We are testing the requirement for p53-mediated transactivation in keratinocyte models of differentiation and neoplastic progression, utilizing transgenic mice expressing a transactivation-deficient p53 mutant under control of the endogenous promoter. We have established that keratinocytes expressing this mutant p53, which retains the ability to bind DNA, behave identically to p53 null keratinocytes in a model of tumor progression, supporting a critical role for p53 mediated transcription in its tumor suppressor activity. The contribution of p53 mediated gene transcription to other stress related responses is currently under study. It has recently become clear that p53 is a member of a gene family, including p63 and p73, which shares sequence homology in the transactivation, DNA binding, and oligomerization domains. The p63 and p73 genes are expressed as multiple isoforms displaying both overlapping and opposing functions. Isoforms lacking the p53 homologous transactivation domain can act in a dominant negative manner to block p53 mediated transcription. p63 expression is restricted to epithelial tissues and the gene is amplified in several human cancers. We have established that a particular p63 isoform, DNp63alpha, which has been shown to block p53 transcriptional activity, can also activate p53 responsive genes. This activity is independent of p53 and appears to be specific for epithelial cells, suggesting a unique role for this isoform in epithelial biology consistent with its restricted expression pattern. We are further defining the differential expression and functional significance of specific p63 isoforms in squamous epithelium under varying physiologic conditions.